config.ru

require 'rubygems'
require 'rack'
require 'open-uri'

class Static
  def call(env)
    [200, {"Content-Type" => "text/html"}, content]
  end

  def content
    open('https://s3.amazonaws.com/yannick-io/index.html')
  end
end

run Static.new

Gemfile

source 'https://rubygems.org'

gem 'puma'
gem 'rack'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    puma (2.10.2)
      rack (>= 1.1, < 2.0)
    rack (1.6.0)

PLATFORMS
  ruby

DEPENDENCIES
  puma
  rack

index.html

<h1>Hello from s3, YO</h1>

Procfile

web: puma -p $PORT -e $RACK_ENV